The present invention relates to films used in packaging food products, and more particularly, to methods and apparatus for making perforated tubular food casings used in processing and packaging meat and sausage products.
It has been common practice to prepare both edible and nonedible webs of tubular food casings for use in the meat packing industry. The nonedible type casings, particularly those fabricated from stronger fibrous reinforced films are especially useful in packaging whole hams, ham butts, picnics as well as bolognas and other larger size sausage and meat products. Generally, fibrous reinforced films are prepared by saturating a paper or other tubular web with a continuous matrix of a film forming polymer, like viscose which is then regenerated, washed, plasticized and dried. As part of the final manufacturing process fibrous food casings are frequently prestuck by perforating with either multiple needle or circular punch size holes which allow venting of the casing during stuffing operations. Generally, perforations provide for shorter stuffing cycles since air can be more readily discharged through the casing sidewall. The perforations also operate to enhance and facilitate drainage of any water, fat or jelly pockets which might otherwise form during or after processing of the meat product.
Although prestuck food casings are well established in the industry, previous methods and equipment for their manufacture have not been totally satisfactory. Heretofore, food films were perforated, for example, with multiple needle-like or flat faced, sharp edged punches mounted circumferentially over the face of a roll, such as described in U.S. Pat. Nos. 3,779,285; 3,126,777 and 3,760,671. In each case, the perforating roll interfaces with a counter-rotating backup roll having a resilient, but substantially solid surface free of complimentary grooves on its outer coverings. Consequently, the punches, needles, etc., perforate by initially compressing the film against the solid surface of the backup roll. This has a tendency to form perforations with frayed or uneven edges which may result in tubular film having an unacceptably high incidence of rupturing. Furthermore, after the film is perforated the needles or punches then make contact with the backup roll reducing their useful life expectancies.
In manufacturing prestuck food films the size and shape of the perforations can be critical. For instance, food casings having vent holes which are either undersized, flared or have frayed edges experience statistically higher failure rates from rupturing during stuffing. Offsetting this problem usually means slower stuffing cycles due to extended venting times. In addition, poor drainage can occur with undersized perforations leading to unappetizing fatty-gel deposits developing between the outer surface of the product and the inner sidewall of the casing. Similar undesirable results can also arise when perforations are oversized, in which case meat emulsion and desirable natural juices can be lost from the package.
In addition to the foregoing, conventional methods of presticking newer premoisturized, ready-to-stuff fibrous casings, which eliminate the need for soaking before filling, do not provide consistently acceptable results. It has been found that presticking premoisturized type casings using known perforating methods, such as needles or punches in some instances form oversized, off-spec perforations or holes with flared edges. Such deviations in the dimensions of perforations in prestuck films can occur through variations in film velocity and lack of synchronization between the film speed through the presticker apparatus and velocity of the perforating rollers occurring, for instance, during start-up and shutdown of the production line. That is, even small variations in film speed or perforation roll velocity can result in off-spec or even torn film. Similar problems may also occur with enlarged perforations with initial entry of the perforating device into the film where the needle or punch pivots in an arc and expands the size of the hole when exiting the perforation. Accordingly, there is a need for improved methods and apparatus for making prestuck food casing with more uniform size holes and with minimal tearing of the film.